The quality of direct-view displays based on conventional technologies such as Cathode Ray Tube (CRT), plasma, Organic Light Emitting Diode (OLED) and direct-view Liquid Crystal Display (LCD), can suffer from the effects of ambient light. Light from extraneous sources such as lighting and windows may impinge on the screen and reflect toward viewers in a manner that can reduce the contrast and viewability of the image. Plasma displays in particular are prone to this problem owing to their relatively low brightness compared with CRT and LCD.
Various approaches to solving this problem have been deployed over the years. For example, the glass envelope on CRT television screens is tinted gray to improve image contrast in ambient light. In these CRT screens, ambient light reaching the screen and reflecting from the internal phosphor and shadow mask layers undergoes absorption twice: once when entering the display and once when exiting. Thus, image light produced by the phosphors undergoes attenuation in proportion to the screen absorbance, while ambient light undergoes twice that amount of attenuation. This can provide an improvement in viewability even though the total image brightness may be reduced, typically by as much as 50%, by the gray glass.
Unlike direct-view displays, rear-projection displays may make use of microlens arrays coupled with apertures in a “black matrix” to form discrete spatial filters capable of providing very high contrast and ambient light rejection. This approach has been used with displays in which imaging light is substantially collimated prior to passing through the screen.
Applying this method to direct-view screens having a Lambertian-like light distribution (generally covering a full hemisphere of view angles) may have the undesirable effect of cutting out a substantial portion of the light, with a loss of brightness. In particular, due to the limited acceptance angle of the microlens/aperture combination with a finite aperture size, such a combination can only pass light incident within a specified range of input angle. Light arriving outside this range misses the aperture and, in the case of a black matrix aperture layer, is absorbed. Since direct-view screens such as plasma and LCD televisions have Lambertian-like light distributions, spatial filtering of the display using a lens/aperture combination that is used for rear-projection displays can cause dramatic and unacceptable loss of brightness.
Another potential problem of using micro-optical films on direct-view displays is the difficulty in mounting such films. Conventional adhesive methods may not be appropriate because the adhesive used can cause index-matching problems with the micro-optical element, which may dramatically reduce or even eliminate the optical power of that element. Creating an air-gap between the display and film may be undesirable due to potential compromise of mechanical rigidity and stability. Thus, even if such a micro-optical film could be made to pass light efficiently, it may still pose difficulties in assembly.